Microstructure and properties of Ni-Ti based gradient laser cladding layer of Ti6Al4V alloy by laser powder bed fusion

An economical gradient layer was successfully prepared on Ti6Al4V alloy produced by laser powder bed fusion (L-PBF) using only composition design, gravity segregation, and laser cladding of micrometer-sized Ni powder, nanoscale SiC and Y2O3 powders. The microstructure of the layer was systematically studied to analyze the reasons for its improved wear and corrosion resistance. The results indicated that the cladding layer comprised top, middle, lower, and bottom layers. The main phases in top layer were Ti2Ni and β-Ti. Whereas the middle layer is composed of two regions, one with a similar structure to the top layer, and the other is the nickel-enriched area denoted as NEAm where the main phases were Ti2Ni, TiNi, and amorphous. The main phase of the lower layer was TiNi. Owing to the close distance between the bottom layer and the substrate, there was a significant increase of Ti in the bottom layer, and so the bottom layer and the top layer contained the same main phases. The soft phase of TiNi in lower layer would prevent the propagation of cracks that may occur in the top layer. The solidification and formation process of the gradient structure of the cladding layer was analyzed with Scheil model, and the simulation results were consistent with the experimental results. A nanocomposite system consisting of Ti3Si, α-Ti, and β-Ti was found for the first time in this study. The relationship between Ti3Si and α-Ti was coherent, and the relationship between α-Ti and β-Ti satisfied the Burgers orientation relationship. The mechanism of the nanocomposite system generation was that Si was enriched at the grain boundaries after the formation of Ti2Ni, where Ti and Si combined to form Ti3Si, and then Ti3Si was used as nucleating particles to form β-Ti/α-Ti. The α-Ti and β-Ti in the fusion line area at the bottom layer also satisfied the Burgers coherent relationship, indicating excellent bonding strength between cladding layer and substrate. A large number of solute atoms in the solid solution, pinning of second-phase particles, and the amorphous were the fundamental reasons for the substantial improvement in hardness and wear resistance. A small amount of Ni in the top layer increased the polarization of the cathodic reaction, keeping the corrosion current low. In addition, numerous dispersed fine phases in the top layer increased the resistance of charge transfer at the phase interfaces, which was the fundamental reason for the improved corrosion resistance.